Method for processing advanced high strength steel

ABSTRACT

A method of manufacturing an energy absorbing component for a vehicle is provided. The method includes heating a bainitic GENS steel material which has a microstructure including ferrite and bainite to a temperature above the Ac3 temperature to convert a portion of the ferrite and bainite to austenite. The method further includes forming while cooling the heated steel blank into a component in a temperature controlled steel die. During the cooling step, the steel material is cooled to a temperature below the Ms temperature to form retained austenite. A portion of the austenite transforms to martensite and bainite during the forming and cooling step. The method can further include heating the component to a temperature above the Ms temperature after the forming and cooling step to increase energy absorption characteristics. During a crash event, the strain imposed on the component converts retained austenite present in the component to martensite.

CROSS-REFERENCE TO RELATED APPLICATIONS

This PCT International Patent Application claims the benefit of andpriority to U.S. Provisional Patent Application Ser. No. 63/026,230filed on May 18, 2020, titled “Method For Processing Advanced HighStrength Steel,” the entire disclosure of which is hereby incorporatedby reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to processing steel, a method ofmanufacturing a component formed of steel, and components formed ofsteel, such as energy absorbing components for vehicle applications.

2. Related Art

This section provides background information related to the presentdisclosure which is not necessarily prior art.

Energy absorbing components, such as structural components for vehicleapplications, are oftentimes formed of steel. Energy absorption is theproduct of strength and ductility, and manufacturability requires goodformability and weldability. Thus, the energy absorbing componentsformed of steel preferably have a good combination of strength,ductility, weldability, and formability.

There are several types of steel materials used to manufacture energyabsorbing components. Many traditional energy absorbing components areformed of a steel material referred to as boron steel. A traditionalprocess of forming a component from boron steel includes heating a sheetformed of the boron steel to a defined elevated temperature and for atime period that enables the formation of a face-centered cubiccrystallographic phase referred to as austenite. The austenitic steelsheet is then transferred to a temperature-controlled steel die. Ahydraulic press forms the component and achieves a desired profile. Thehydraulic press applies the force required to form the desired profileand controls the rate of heat transfer, to achieve the desired coolingrate. The cooling rate and alloy composition of the boron steel causes aphase transformation of the low strength austenite to either a highstrength martensite phase or pearlitic microstructure. The criticalcooling rate is based on the alloy composition. The combination of alloycomposition and cooling rates imposed by conventional hot stampprocessing of boron steel does not result in retained austenite.

Emerging energy absorbing components are currently comprised of bainiticquenched and partitioned steels referred to as bainitic GEN3 steels. TheGEN3 steels are a commercially available series of advanced highstrength steel (AHSS) which have a high strength and ductility, which isassociated with the bainitic microstructure. There are various grades ofGEN3 steels, based on alloy composition and thermal processing. Thetransformation of austenite to bainite in the steel is typicallyaccomplished at the rolling mill and is referred to as a quench andpartition heat treatment. The components formed of the GEN3 steels areformed at room temperature.

There are advantages and disadvantages associated with the boron steeland the GEN3 steels described above.

For example, the hot stamped boron steel exhibits a higher energyabsorption characteristic than the GEN3 steel. The forming tonnagerequired to form the boron steel at an elevated temperature is lowerthan that required for the GEN3 steel at room temperature. In addition,the cost of a boron steel sheet is less than a GEN3 steel sheet.

However, post-formed hot stamped boron steel has a relatively lowductility, which limits commercial applications to crash-formedstrength-based bending applications, which do not include flange designfeatures. The flange design features increase design efficiency andfacilitate attachment to other components. The post-formed strength andductility characteristics of components formed of the hot stamped boronsteel necessitate the use of lasers to trim the stamped components. Theprocessing and manufacturing costs of the hot stamped boron steelcomponents are high due to the capital costs, operating costs, and floorspace allocation associated with blank preheat furnaces, hydraulicpresses, and laser trim equipment typically used to manufacture thecomponents. The manufacturing and processing costs are greater thanthose associated with the GEN3 steels, due to the increased capital andoperating cost associated with inline solution heat treat of the boronsteel sheet prior to the forming operation, use a hydraulic presscapable of stopping at the bottom to achieve the required transformationcooling rate, and the laser-based trim processes required to trim thestamped components formed of the boron steel. In addition, thepost-formed microstructure of the conventional hot stamped boron steeltypically includes martensite, but does not include retained austenite.Thus, the boron steel components lack a post-forming work hardeningresponse associated with the transformation of retained austenite in thepost-formed matrix.

As indicated above, the quenched and partitioned GEN3 steels, comprisedof a combination of bainite and retained austenite, have improvedformability and ductility relative to martensitic hot stamped steelenabling the ability to form flange features to increase the designefficiency of the component. The quenched and partitioned GEN3 steelsalso have the advantage of reduced processing costs, relative to the hotstamped boron steel. The reduced processing costs are typicallyassociated with processing at room temperature, reduced cycle timerelated to the use of a higher speed mechanical press, avoidance ofdwell time associated with transformation cooling, and feasiblesecondary operations (restrike, trim, flange and pierce) which areperformed in-line to the forming operation.

However, the GEN3 steels are typically more costly than the boron steel.The post-formed dimensional repeatability of the GEN3 steel stampedcomponents is low relative to the hot stamped boron steel and other highstrength steel alloys stamped at room temperature. The reduceddimensional repeatability is related to spring back. The post-formedtotal energy absorption characteristics of the GEN3 steel is lowerrelative to boron steel. The strength of the GEN3 steel during theforming operation is high relative to the hot stamped boron steel, whichlimits the size (area) or number of GEN3 steel parts formed for a givenpress tonnage. Increased press tonnage is required relative to the hotstamped boron steel. In addition, bainitic GEN3 steel does not exhibit awork hardening characteristic due to the lack of retained austeniteafter the forming operation.

A commercially available series of GEN3 steel is an austenitic advancedhigh strength steel (AHSS) referred to as austenitic GEN3 TRIP steel.TRIP steels leverage the strength and ductility associated with thetransformation to austenite to martensite (known as the TRIP effect) toenhance formability and strength characteristics, as a result of strainimposed during the forming process.

There is a continuing desire to further develop and refine steels usedto form energy absorbing components, such as those used in vehicleapplications. Objectives include increasing product design efficiency byenabling the capability to form flange features; avoiding cost byenabling inline restriking, trimming, flanging, and piercing operations;avoiding cost associated with secondary laser processing operations;improving post-formed dimensional repeatability, associated withspringback of GEN3 steel stamped components; avoiding cost by cycle timereduction; avoiding capital cost associated with use of a mechanicalpress compared to a hydraulic press; avoiding cost by cycle timereduction, specifically by eliminating cooling rate-dependenttransformation events; reducing capital equipment, operational costs,and floorspace required by preheat ovens, presses, and secondary trimoperations; and increasing energy absorption associated with TRIPenabled work hardening and plasticity characteristics during a straininduced crash event.

SUMMARY

One aspect of the invention provides a method for processing steelmaterial, such as material used to form an energy absorbing componentfor a vehicle. The method comprises heating a steel material to atemperature above an upper critical temperature (Ac3) of the steelmaterial. The steel material has a microstructure which includes ferriteand bainite, and the heating step includes converting a portion of theferrite and bainite to austenite. The method further includes formingthe steel material into a component after the steel material is heatedto the temperature above the upper critical temperature (Ac3) of thesteel material. The steel material is cooled during the forming step,and a portion of the austenite transforms to martensite and bainiteduring the forming step.

Another aspect of the invention provides a component formed of the steelmaterial, for example an energy absorbing component for a vehicle. Thesteel material includes iron in an amount of 91.95 to 98.55 wt. %,carbon in an amount of 0.15 to 0.3 wt. %, manganese in an amount of 1.5to 2.5 wt. %, silicon in an amount of 0.6 to 1.6 wt. %, chromium in anamount of 0.55 to 0.65 wt. %, copper in an amount of 0.0 to 1.0 wt. %,nickel in an amount of 0.0 to 1.0 wt. % and aluminum in an amount of 0.0to 1.0 wt. %, based on the total weight of the steel material. The steelmaterial also includes bainite and martensite.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an energy absorbing component formed of a steelmaterial according to an example embodiment;

FIG. 2 illustrates a quench and partition process wherein a steelmaterial is heated above the Ac3 temperature of the steel material, diequenched in a heated die to a temperature between the M_(s) and M_(f)temperature of the steel material, and then heated to an elevatedtemperature to increase energy absorption.

FIG. 3 illustrates a quench and temper process wherein a steel materialis heated above the Ac3 temperature of the steel material, die quenchedin a steel die to a temperature below the M_(s) and M_(f) temperaturesof the steel material, and reheated to an elevated temperature toincrease energy absorption.

FIG. 4 is a table showing ultimate tensile strength (TS), yield strength(YS), and elongation (E) for a steel material in an as-receivedcondition and steel materials processed according to exampleembodiments.

FIG. 5 is a graph of phase distribution and temperature for a steelmaterial according to an example embodiment.

DETAILED DESCRIPTION

One or more of the above objectives are achieved by embodiments of theinvention. In general, the subject embodiments are directed to a methodfor processing advanced high strength steel (AHSS). However, the exampleembodiments are only provided so that this disclosure will be thorough,and will fully convey the scope to those who are skilled in the art.Numerous specific details are set forth such as examples of specificcomponents and methods, to provide a thorough understanding ofembodiments of the present disclosure. It will be apparent to thoseskilled in the art that specific details need not be employed, thatexample embodiments may be embodied in many different forms and thatneither should be construed to limit the scope of the disclosure. Insome example embodiments, well-known processes, well-known devicestructures, and/or well-known technologies are not described in detail.

According to example embodiments, the method includes processing of theadvanced high strength steel (AHSS) referred to as bainitic GEN3 steel.Bainitic GEN3 steel typically comprises iron in an amount of 91.95 to98.55 wt. %, carbon in an amount of 0.15 to 0.3 wt. %, manganese in anamount of 1.5 to 2.5 wt. %, silicon in an amount of 0.6 to 1.6 wt. %,chromium in an amount of 0.55 to 0.65 wt. %, copper in an amount of 0.0to 1.0 wt. %, nickel in an amount of 0.0 to 1.0 wt. % and aluminum in anamount of 0.0 to 1.0 wt. %, based on the total weight of the steelmaterial. According to a specific example, the composition of the steelmaterial, demonstrated at lab scale, comprises iron in an amount of96.03 wt. %, carbon in an amount of 0.22 wt. %, manganese in an amountof 2.35 wt. %, silicon in an amount of 0.6 wt. %, and aluminum in anamount of 0.8 wt. %, based on the total weight of the steel. Themicrostructure of the steel includes bainite, typically in an amount ofat least 75 vol. %, based on the total volume of the steel material. Theremainder of the steel material includes ferrite. The process beginswith a blank formed of the steel material, which is typically in theform of a sheet. The following example embodiments will refer to thesteel sheet, however, the steel material could comprise other shapes.

The bainitic steel material is heated to a temperature above the uppercritical temperature (Ac3) of the steel material. The Ac3 temperature isdefined at the temperate at which the ferrite and bainite phases of thesteel material transform to austenite. Thus, during the heating step, aportion of the ferrite and bainite transform to austenite. Typically,for the GEN3 steel material, the temperature above the Ac3 temperatureranges from 850° C. to 900° C. The Ac3 temperature for the bainitic GEN3steel disclosed above is 850 ° C. However, the Ac3 temperature varies bycomposition, and Ac3 kinetics are slow. Heating above the Ac3temperature reduces the time required to achieve a microstructure whichis 100% austenite.

The specific fraction of ferrite, bainite and austenite in the steelmaterial after the heating step is dependent on a phase equilibrium attemperature for the specific composition of the steel material. Thefraction of ferrite, bainite and austenite in the steel sheet is alsodependent on the temperature history of the steel sheet prior to formingand the specific composition of the steel material.

Next, the steel sheet, which was previously heat treated to atemperature above the Ac3 temperature, typically 850° C. to 900° C., isformed into a component 10 having a desired shape. The forming step ispreferably conducted in a temperature controlled steel die using aforming press, preferably a mechanical press. The method also includescooling the steel material during and possibly after the forming step.The temperature of the steel die ranges from 100° C. to 360° C. whileforming the steel material into the desired shape. The temperature ofthe steel material itself during the forming step ranges from 900° C. toa temperature ranging between 100° C. to 360° C.

A high fraction percentage of the austenite is transformed to martensiteand bainite during the forming process, as a result of the rate of heattransfer imposed by the forming process. The transformation of theaustenite to a combination of bainite and/or martensite during theforming step reduces the forming tonnage required, improves formability,reduces dimensional variance by improving dimensional repeatabilityassociated with spring back, and increases the strength of the formedcomponent 10. An example of the component 10 is shown in FIG. 1 .According to this example, the component 10 is a B-pillar between apassenger and driver door of a vehicle.

As indicated above, during and possibly after the forming step, themethod preferably includes cooling the steel material and/or shapedcomponent in the die, for example by quenching. The steel materialand/or component is cooled to a temperature below the M_(s) temperature.After cooling to a temperature below the M_(s) temperature, the methodpreferably includes heating or tempering the component to a temperatureabove the M_(s) temperature in the die. The M_(s) temperature is thetemperature at which the formation of martensite in the steel materialbegins, and the M_(f) temperature is the temperature at which theformation of martensite in the steel material finishes. Regulating thetemperature of the die during and after the forming step controls theamount of martensite, bainite, and retained austenite in the componentand thus is able to tailor the energy absorption, weldability, and/ordeformation characteristics in specific regions of the component.

The cooling step typically includes forming retained austenite in thecomponent. The retained austenite is maintained in a matrix of bainiteand martensite. For example, greater than 0 and up to 15 volume % of theaustenite present in the steel material prior to the forming step may beretained in the matrix of bainite and martensite after the cooling step.The percentage of retained austenite in the post-formed steel sheet isdependent on the temperature of the form die, cooling rate, strainimposed during the forming process and the specific steel composition.

The amount of retained austenite present in the component after formingis the result of diffusion-related transformation kinetics relative tothe martensite start temperature (M_(s)) and martensite finish (M_(s))temperature range. The M_(s) temperature for the steel compositiondisclosed above is approximately 350° C. to 360° C. and the M_(f)temperature is approximately 135° C. to 145° C. The percentage ofretained austenite in the component ranges from 0% to 15% based onstability of the austenite during cooling determined by the cooling ratebelow the M_(s) temperature. Austenite stability and the relativepercentage of bainite versus martensite present in the formed componentis determined by the cooling rate below the M_(s) temperature which isinfluenced by the temperature of the steel die used to form thecomponent.

The method can further including heating or tempering the steelcomponent after the cooling and forming steps.

According to one embodiment, the temperature of the steel component inthe steel die is controlled and is kept at temperature between the M_(s)and M_(f) temperatures of the steel material after the forming step, andthen the steel component is heated to a temperature above the M_(s)temperature for a defined period of time. FIG. 2 illustrates a quenchand partition process wherein the steel material is heated above the Ac3temperature, die quenched in a heated die to a temperature between theM_(s) and M_(f) temperatures, which are specific to the steel materialcomposition, and then heated to an elevated temperature to increaseenergy absorption.

According to another embodiment, the temperature of the steel componentis controlled and kept at a temperature below the M_(f) temperatureprior to heating the steel component to a temperature above the M_(s)temperature for a defined period of time. FIG. 3 illustrates a quenchand temper process wherein the steel material is heated above the Ac3temperature, die quenched in a steel die to a temperature below theM_(s) and M_(f) temperatures, which are specific to the steel materialcomposition, and reheated to an elevated temperature to increase energyabsorption. The cooling and reheating steps are conducted to increaseenergy absorbing properties of the steel component. Various otherheating, tempering, quenching, partitioning, and/or austenitizing stepscan be conducted on the steel component after the forming step toincrease energy absorbing properties of the steel component.

The composition of the steel material of the finished component stillincludes iron in an amount of 91.95 to 98.55 wt. %, carbon in an amountof 0.15 to 0.3 wt. %, manganese in an amount of 1.5 to 2.5 wt. %,silicon in an amount of 0.6 to 1.6 wt. %, chromium in an amount of 0.55to 0.65 wt. %, copper in an amount of 0.0 to 1.0 wt. %, nickel in anamount of 0.0 to 1.0 wt. % and aluminum in an amount of 0.0 to 1.0 wt.%, based on the total weight of the steel material.

According to an example embodiment, the method includes heating thesteel material to a temperature above the Ac3 temperature, preferably toa temperature of 900° C. The steel material is then cooled during theforming process in a steel die, preferably controlled to a temperatureof 100° C. to 350° C. The cooling rate of the steel material below theM_(s) temperature is greater than 10° C./second, preferably 50°C./second. The formed component is then reheated to a temperature abovethe M_(s) temperature, preferably to a temperature range of 360° C. to400° C.

FIG. 4 is a table showing the ultimate tensile strength (TS), yieldstrength (YS), and elongation (E) for a steel component in theas-received condition; and steel components which have been austenized,quenched and partitioned; austenized and quenched; and austenized,quenched, and tempered according to example embodiments. FIG. 5 includesa graph of phase distribution and temperature for a steel materialaccording to an example embodiment.

The process can further include restriking, trimming, flanging, and/orpiercing operations on the finished formed steel component. If thefinished formed component is used in a vehicle application and includesa fraction percentage of retained austenite, then during a possiblecrash event the formed component is subjected to strain which transformssome of the retained austenite to martensite. The transformation of theretained austenite to martensite during the crash event increasesstrength and energy absorption characteristic of the component.

As indicated above, the process and finished component formed by theprocess described above provides numerous advantages. The transformationof austenite to a combination of martensite, bainite and/or retainedaustenite addresses the need to improve dimensional repeatability,formability, forming tonnage requirements, and energy absorptioncharacteristics, relative to GEN3 bainitic steel formed at roomtemperature. The transformation of austenite to a combination ofmartensite, bainite and retained austenite also addresses the need toreduce manufacturing costs, enables use of a mechanical press, andincreases design efficiency relative to hot stamped boron steelcomponents. The transformation of retained austenite to martensiteduring a strain event imposed during a crash addresses the need toimprove energy absorption characteristics relative to GEN3 bainiticsteel.

The steel component of the present disclosure also provides enhancedformability due to the presence of retained austenite and transformationof the austenite to martensite during the forming process. Thedimensional characteristics associated with the steel component are alsoenhanced due to the presence of the retained austenite and thetransformation of the austenite to martensite during the formingprocess. The post-formed energy absorption characteristics of the steelcomponent are greater than GEN3 boron steel due to the transformation ofa portion of austenite to martensite during the forming event and thetransformation of the retained austenite to martensite during a crashevent. The cost associated with the manufacture of the steel componentis less than boron steel due to reduced heating requirements and use oflower cost trimming methods. The design efficiency of the steelcomponent is greater than hot stamped boron steel due to the ability toform flange features.

It should be appreciated that the foregoing description of theembodiments has been provided for purposes of illustration. In otherwords, the subject disclosure it is not intended to be exhaustive or tolimit the disclosure. Individual elements or features of a particularembodiment are generally not limited to that particular embodiment, but,where applicable, are interchangeable and can be used in a selectedembodiment, even if not specifically shown or described. The same mayalso be varies in many ways. Such variations are not to be regarded as adeparture from the disclosure, and all such modifications are intendedto be included within the scope of disclosure.

1. A method for processing steel material, comprising the steps of:heating a steel material to a temperature above an upper criticaltemperature (Ac3) of the steel material, and the steel material having amicrostructure which includes ferrite and bainite; the heating stepincluding converting a portion of the ferrite and bainite to austenite;forming the steel material into a component after the steel material isheated to the temperature above the upper critical temperature (Ac3);and cooling the steel material during the forming step, wherein aportion of the austenite transforms to martensite and bainite during theforming step.
 2. The method of claim 1, wherein the heating stepincludes heating the steel material to a temperature greater than 850°C.
 3. The method of claim 1, wherein the steel material includes iron inan amount of 91.95 to 98.55 wt. %, carbon in an amount of 0.15 to 0.3wt. %, manganese in an amount of 1.5 to 2.5 wt. %, silicon in an amountof 0.6 to1.6 wt. %, chromium in an amount of 0.55 to 0.65 wt. %, copperin an amount of 0.0 to 1.0 wt. %, nickel in an amount of 0.0 to 1.0 wt.% and aluminum in an amount of 0.0 to 1.0 wt. %, based on the totalweight of the steel material.
 4. The method of claim 1, wherein themicrostructure of the steel material, prior to the heating step,includes bainite in an amount of at least 75 vol. %, based on the totalvolume of the steel material.
 5. The method of claim 1, wherein theforming step is conducted in a steel die while the steel die is at atemperature of 100° C. to 360° C.
 6. The method of claim 1, wherein thecooling step includes cooling the steel material from the temperatureabove the Ac3 temperature to a temperature below a martensite start(M_(s)) temperature of the material.
 7. The method of claim 6, whereinthe Ac3 temperature is greater than 850° C. and the temperature belowthe M_(s) temperature is from 100° C. to 350° C.
 8. The method of claim1, wherein the cooling step includes forming retained austenite, whereinthe retained austenite is maintained in a matrix of the bainite andmartensite
 9. The method of claim 8, wherein up to 15% of the austenitepresent in the steel material prior to the forming step is maintained asthe retained austenite in the matrix of the bainite and martensite. 10.The method of claim 1, wherein the cooling is conducted at a rate ofless than 50° C./second.
 11. The method of claim 1 including heating thecomponent to a temperature above the M_(s) temperature of the steelmaterial after the forming and cooling step.
 12. The method of claim 1including restriking, trimming, flanging, and/or piercing componentafter the forming step.
 13. The method of claim 1, wherein the formingstep is conducted in a die, and the method further includes regulatingthe temperature of the die during and/or after the forming step tocontrol the amount of martensite, bainite, and retained austenite in thecomponent and thus tailor the energy absorption, weldability, and/ordeformation characteristics in specific regions of the component. 14.The method of claim 1, wherein the forming step includes shaping thesteel material into the component having the shape of an energyabsorbing component for a vehicle.
 15. A component, comprising: a steelmaterial including iron in an amount of 91.95 to 98.55 wt. %, carbon inan amount of 0.15 to 0.3 wt. %, manganese in an amount of 1.5 to 2.5 wt.%, silicon in an amount of 0.6 to 1.6 wt. %, chromium in an amount of0.55 to 0.65 wt. %, copper in an amount of 0.0 to 1.0 wt. %, nickel inan amount of 0.0 to 1.0 wt. % and aluminum in an amount of 0.0 to 1.0wt. %, based on the total weight of the steel; and the steel materialincluding bainite and martensite.
 16. The method of claim 1, whereinbefore the heating step, the microstructure of the steel includesbainite in an amount of at least 75 vol. %, based on the total volume ofthe steel material, and a remainder of the microstructure includesferrite.
 17. The method of claim 1, wherein the component is a B-pillar.18. The method of claim 1, wherein the cooling is conducted at a rate ofgreater than 10° C./second.
 19. The component of claim 15, wherein thesteel material includes retained austenite in a matrix of bainite andmartensite.
 20. The component of claim 15, wherein the component is aB-pillar.